Directional SiO2 etch using plasma pre-treatment and high-temperature etchant deposition

ABSTRACT

Methods for processing a substrate are described herein. Methods can include positioning a substrate with an exposed surface comprising a silicon oxide layer in a processing chamber, biasing the substrate, treating the substrate to roughen a portion of the silicon oxide layer, heating the substrate to a first temperature, exposing the exposed surface of the substrate to ammonium fluoride to form one or more volatile products while maintaining the first temperature, and heating the substrate to a second temperature, which is higher than the first temperature, to sublimate the volatile products.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of co-pending Patent Cooperation TreatyApplication Ser. No. PCT/US13/060195, filed Sep. 17, 2013, which claimsthe benefit of U.S. Provisional Patent Application Ser. No. 61/709,107,filed Oct. 2, 2012, and of U.S. Provisional Patent Application Ser. No.61/874,783, filed Sep. 6, 2013. Both are herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Technology described herein relates to directional etching of nativeoxides. Specifically, technology described herein relates topretreatment of an oxide surface to selectively etch the oxide surfaces.

2. Description of the Related Art

With the increase in transistor density and subsequent decrease in thecross-sectional dimensions of device nodes, which can be less than 22nm, pre-clean of native oxides is of particular importance. Pre-cleancan include pre-contact clean or pre-silicide clean which requiresremoval of oxides from the bottom of vias or trenches of narrowingcross-sectional dimensions. As critical dimension of semiconductordevices decreases, distances between neighboring features formed on asemiconductor substrate are also shortened. Thus, it is important tocontrol etching between vias and trenches during precleaning to preventdamaging nearby features.

Current precleaning techniques generally includes a conformal etch ofthe substrate to remove the native oxides, such as SiO₂, prior todeposition of silicides or other contacts. However, a standard conformaletch can lead to excessive cross-sectional enlargement of vias andtrenches thus creating possible leakage and ultimate device failure.Other precleaning techniques such as sputter etching remove nativeoxides from trench or contact bottom surfaces. However, the sputteringprocess can also lead to redeposition of field oxides at the via ortrench opening. The redeposited oxides create an overhang at vias andtrenches openings leading to poor subsequent contact fill.

Thus, methods are needed to preferentially etch from the bottom surfacesof features to prevent damage to features during precleaning.

SUMMARY OF THE INVENTION

Embodiments described herein generally relate to selective etching ofnative silicon oxides. In one embodiment, a method can includepositioning a substrate in a processing chamber, the substratecomprising an exposed surface, one or more features formed in theexposed surface, the features comprising a bottom surface and an oxidelayer formed on the exposed surface; biasing the substrate; exposing thesubstrate to a low energy inert plasma to selectively form physically orchemically activated material on the exposed surface and bottom surfacesof the features; heating the substrate to a first temperature; exposingthe substrate to a processing gas comprising ammonium fluoride (NH₄F) orNH₄F(HF) to form one or more volatile products on the exposed surfaceand bottom surfaces of the features; and heating the substrate to asecond temperature, which is higher than the first temperature, tosublimate the one or more volatile products.

In another embodiment, a method can include positioning asilicon-containing substrate in a processing chamber, thesilicon-containing substrate including an exposed surface, one or morefeatures formed in the exposed surface and a layer of surfacecontaminants formed on the exposed surface; cooling the substrate to afirst temperature; exposing the exposed surface of the substrate toammonium fluoride (NH₄F), (NH₄F)HF or combinations thereof at the firsttemperature; biasing the substrate; exposing the substrate to a lowenergy inert plasma to selectively form one or more volatile products onthe exposed surface and bottom surfaces of the features; exposing thesubstrate to low pressure at a second temperature, wherein a non-reactedNH₄F is sublimated from the exposed surface of the substrate; andheating the substrate to a third temperature, which is higher than thefirst and second temperature, to sublimate the one or more volatileproducts.

In another embodiment, a method can include positioning a semiconductorsubstrate in a processing chamber, the semiconductor substrate caninclude an exposed surface, one or more features formed in the exposedsurface and an oxide layer formed on the exposed surface; biasing thesubstrate; performing a contaminant removal process which can includeexposing the substrate to a low energy inert plasma to selectively formphysically or chemically activated material on the top and bottomsurfaces of the features, heating the substrate to a first temperature,exposing the surface of the substrate to a processing gas comprisingammonium fluoride (NH₄F) or NH₄F(HF) to form one or more volatileproducts on the top and bottom surfaces of the features and heating thesubstrate to a second temperature, which is higher than the firsttemperature, to sublimate the one or more volatile products; andrepeating the contaminant removal process one or more times.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic top-view diagram of an illustrative multi-chamberprocessing system useable with embodiments described herein.

FIG. 2 is a diagram of a method for directional etching according to oneembodiment.

FIG. 3 depicts etch rate of the silicon oxide as a function of pedestaltemperature according to one embodiment.

FIGS. 4A-4C are graphical representations of a substrate etchedaccording to one or more embodiments.

FIGS. 5A and 5B show a transmission electron microscope (TEM) view of asubstrate directionally etched as described herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Methods for removing native oxides are described herein. Precleaning ofsurfaces in vias and trenches can lead to etching of sidewalls andsubsequent reduction of cross sectional dimensions of the solid wallsseparating the trenches or vias being cleaned from adjacent features ina semiconductor device. This reduction of cross-sectional dimensions canlead to device failure. The embodiments described herein allow fordirectional etching of surfaces to remove native oxides from bottoms ofvias and trenches while preserving the cross-sectional dimensions of thevia or trench. By employing a directional pretreatment of the nativeoxide surface prior to conformal exposure to an etchant at elevatedtemperatures, the affected surface is prepared for preferential etchingwhile the untreated surface is nominally etched due to equilibriumadsorption/desorption. The invention is more fully explained withreference to the figures below.

FIG. 1 is a schematic top-view diagram of an illustrative multi-chamberprocessing system 200 that can be adapted to perform processes asdisclosed herein having one or more processing chambers coupled thereto,such as processing chamber 100. The system 200 can include one or moreload lock chambers 202, 204 for transferring substrates into and out ofthe system 200. Typically, since the system 200 is under vacuum, theload lock chambers 202, 204 can “pump down” the substrates introducedinto the system 200. A first robot 210 can transfer the substratesbetween the load lock chambers 202, 204, and a first set of one or moresubstrate processing chambers 212, 214, 216, 100 (four are shown).Processing chambers 100 and 216 are can be degas chambers to used topre-heat and drive moisture out of incoming substrates. Processingchambers 212 and 214 can be preclean chambers. The positions of theprocessing chamber 212 and 214 utilized to perform the preclean processrelative to the other chambers is for illustration, and the position ofthe processing chamber 212 and 214 may be optionally be switched withany one of the processing chambers 212, 214, 216 or 100, if desired.Further, one or more of the chambers can be positioned at empty position250, if desired.

The first robot 210 can also transfer substrates to/from one or moretransfer chambers 222, 224. The transfer chambers 222, 224 can be usedto maintain ultrahigh vacuum conditions while allowing substrates to betransferred within the system 200. A second robot 230 can transfer thesubstrates between the transfer chambers 222, 224 and a second set ofone or more processing chambers 232, 234, 236, 238. Similar toprocessing chambers 212, 214, 216, 100, the processing chambers 232,234, 236, 238 can be outfitted to perform a variety of substrateprocessing operations.

The processing chamber 212 or 214 may be configured to remove nativeoxides or other contaminants from a substrate surface prior to forming apre-contact layer or metal silicide layer on the substrate, such asnative oxides from the bottoms of features disposed on a substrate. Theprocessing chamber 212 or 214 can be particularly useful for performingthe plasma assisted dry etch process (i.e. the “preclean process”). Theprocessing chamber 212 or 214 may be a Preclean PCII, PCXT or etchchambers which are available from Applied Materials, Inc., located inSanta Clara, Calif. It is noted that other chambers available from othermanufactures may also be utilized to practice the present invention.

After the preclean process is performed in the processing chamber 212 or214, the substrate may further be transferred to any of the processingchambers 100, 212, 214, 216, 232, 234, 236, 238 disposed in the system200 to perform the second step of the process, such as a SiConi chamberor etch chamber from Applied Materials, Inc located in Santa Clara,Calif.

FIG. 2 is a diagram of a method for directional etching according to oneembodiment. The method 300 can include positioning a substrate withsurface contaminants in the processing chamber, as in 302. Theprocessing chamber can be a processing chamber as described withreference to FIG. 1 or it can be a different processing chamber. Theprocessing chamber should be capable of at least maintaining thesubstrate at a specific temperature, biasing the substrate and formingNH₄F (e.g. creating NH₄F in a plasma). The substrate can be a siliconsubstrate with features formed on the surface. The features can includeone or more of vias and trenches of varying cross-sectional dimensions,such as less than 22 nm. Native oxides may be formed on one or more ofthe surfaces of the substrate, such as a silicon dioxide formed on allexposed surfaces. The native oxides may also be of varying thicknessdependant on the circumstances of formation, such as exposure to theatmosphere.

The method 300 can further include treating the substrate with a lowenergy direct plasma, as in 304. The direct plasma can comprise anyinert gas. Inert gases include noble gases, such as helium or argon. Theinert gas is formed into a plasma of sufficiently low energy so as tonot sputter the substrate.

The direct plasma includes a bias voltage at the wafer surface which canbe due to self-bias, direct application of RF bias energy to thesubstrate wafer support pedestal, or a combination of both. The bias onthe substrate can be of any power, but is preferably below the substratesputter threshold. The substrate sputter threshold can be less than150V, such as less than 75V. The bias can be delivered at varyingfrequencies, such as a bias of 2 Mhz, 13.56 Mhz, 60 Mhz or combinationsthereof. The bias applied to the substrate attracts the ionized gas inthe plasma toward the substrate, where the ionized gas strikes surfaceswhich are perpendicular to the direction of ionized gas movement, suchas the bottom of a via or trench on a substrate. The ionized gas thuschanges the surface by various mechanisms, including mechanicalroughening, forming dangling bonds at surface, changing surface density,or formation of amorphous surfaces. These surface changes prepare thesurface for subsequent directional etching.

The method 300 can further include heating the substrate to a firsttemperature, as in 306. The first temperature can be a temperature from65° C. to 100° C., such as from 70° C. to 100° C., In one or moreembodiments, the substrate can be heated to a temperature above 65° C.,such as between 65° C. and 110° C., by a heating apparatus formed withinthe substrate support member. In one embodiment, the substrate ismaintained at 70° C. In another embodiment, the substrate is maintainedat a temperature of between 70° C. and 110° C.

The method 300 can further include forming ammonium fluoride (NH₄F) toetch the substrate, as in 308. NH₄F can be used in dry etch processesfor removing silicon oxides, SiN, or other materials from wafersurfaces. NH₄F can be formed in-situ from ammonia (NH₃) and nitrogentrifluoride (NF₃) gas mixtures within a processing chamber. Etching SiO₂with NH₄F is generally accomplished on a heated substrate. The substratecan be heated to a temperature above the dew point of NH₄F. The dewpoint will be dependent on process conditions such as processing chamberpressure. An etching gas mixture is introduced to the chamber forremoving silicon oxides on a surface of the substrate. In oneembodiment, ammonia and nitrogen trifluoride gases are then introducedinto the plasma chamber to form the etching gas mixture. The amount ofeach gas introduced into the plasma chamber is variable and may beadjusted to accommodate, for example, the thickness of the oxide layerto be removed, the geometry of the substrate being cleaned, the volumecapacity of the plasma, the volume capacity of the chamber, as well asthe capabilities of the vacuum system coupled to the chamber. The ratioof the etching gas mixture may be predetermined to remove various oxideson the substrate surface. The ratio of gas mixture in the etching gasmixture may be adjusted to preferentially remove the pre-treated oxides,such as native oxides formed on the top and bottom surfaces of thefeatures. In one embodiment, molar ratio of ammonia to nitrogentrifluoride in the etching gas mixture may be set to uniformly removesilicon oxides.

In one embodiment, etching rate of the etching gas mixture may beadjusted by adjusting a flow rate of nitrogen trifluoride whilemaintaining a molar ratio of ammonia and nitrogen trifluoride above apredetermined value. In one embodiment, etching rate may be increased ordecreased by increasing or decreasing the flow rate of nitrogentrifluoride while the ratio of ammonia and nitrogen trifluoride remainsabove about 3:1. In another embodiment, the ratio of ammonia andnitrogen trifluoride can be about 1:1.

The ammonia and nitrogen trifluoride gases can be dissociated intoreactive species in a remote plasma chamber. The dissociated species cancombine to form a highly reactive ammonia fluoride (NH₄F) compoundand/or ammonium hydrogen fluoride (NH₄F.HF) in the gas phase. Thesemolecules react with the substrate surface to be processed. In oneembodiment, an inert carrier gas is first introduced into the plasmachamber, a plasma of the carrier gas is generated, and then the reactivegases, ammonia and nitrogen trifluoride, are added to the plasma.

Not wishing to be bound by theory, it is believed that the etchant gas,NH₄F and/or NH₄F.HF, reacts with the silicon oxide surface to formammonium hexafluorosilicate (NH₄)₂SiF₆, NH₃, and H₂O products. The NH₃,and H₂O are vapors at processing conditions and removed from the chamberby a vacuum pump. A thin film of (NH₄)₂SiF₆ is left behind on thesubstrate surface. This reaction mechanism can be summarized as follows:NF₃+3NH₃→NH₄F+NH₄F.HF+N₂6NH₄F+SiO₂→(NH₄)₂SiF₆+2H₂O+4NH₃(NH₄)₂SiF₆+heat→2NH₃+2HF+SiF₄

After the products are reacted, the non-reacted NH₄F can be sublimatedat low pressure and removed from the chamber so as to not affect furtherprocessing.

The method 300 can further include removing the (NH₄)₂SiF₆ by heatingthe substrate to a second temperature to sublimate volatile byproducts,as in 312. After the thin film is formed on the substrate surface, thesupport member may be elevated to an anneal position in close proximityto a heated gas distribution plate. The heat radiated from the gasdistribution plate may dissociate or sublimate the thin film of(NH₄)₂SiF₆ into volatile SiF₄, NH₃, and HF products. These volatileproducts are then removed from the chamber by the vacuum pump asdescribed above. Typically, a temperature of 75° C. or more is used toeffectively sublimate and remove the thin film from the substrate.Preferably, a temperature of 100° C. or more is used, such as betweenabout 115° C. and about 200° C. Sublimation of solid (NH₄)₂SiF₆ can alsobe accomplished by heating the wafer by increasing the temperature ofthe wafer susceptor used to support the wafer. The wafer heating can beaccomplished in the same process chamber or in another chamber if it ismore efficient to move the heat and sublimation step elsewhere.

The method 300 can further include flowing inert gas to evacuate thevolatile byproducts from the chamber, as in 314. The thermal energy todissociate the thin film of (NH₄)₂SiF₆ into its volatile components istransferred by the gas distribution plate through convection orradiation. In one aspect, the distribution plate is heated to atemperature of between 100° C. and 150° C., such as about 120° C.Further embodiments use a low energy plasma, such as a plasma asdescribed with reference to the pretreatment process, to enhance thesublimation of volatile byproducts. The plasma is delivered to thesurface of the substrate uniformly and at an energy level which will notsputter the oxides form the substrate. By using a low energy plasmawhile simultaneously heating the substrate, it is believed that theactivation energy for sublimation can be reduced. For example, a layerof (NH₄)₂SiF₆ may be of a certain thickness which requires a temperatureof 120° C. over a certain time period to sublimate. By using a lowenergy plasma, the layer of (NH₄)₂SiF₆ can be sublimated at 100° C. overthe same time period or at 120° C. over a shorter time period.

Once the film has been removed from the substrate, the process can beended, as in 316. The processing chamber is purged and evacuated. Theprocessed substrate is then removed from the chamber by lowering thesubstrate member to the transfer position, de-chucking the substrate,and transferring the substrate through a slit valve opening.

Without intending to be bound by theory, it is believed that at elevatedtemperatures and low partial pressures of NH₄F or NH₄F(HF), the chemicaletch rate of SiO₂ without plasma activation as in 304 can be nominal(e.g. approximately zero) because the partial pressure of NH₄F etchantcan be maintained below the partial pressure required for NH₄Fcondensation (wafer is above dew point temperature for NH₄F). During astandard NH₄F etch process, the substrate will be maintained at atemperature less than 40° C., such as a temperature between 25° C. and40° C. In this temperature range, the reaction between the NH₄F and theoxide layer on the substrate is believed to be reaction limited, suchthat higher levels of reactant will lead to increased and uniformetching of the oxide layer. When the wafer temperature is raised abovethe dew point of NH₄F, the adsorption rate is closer to the adsorptionrate of NH₄F on the substrate surfaces not exposed to plasma activationas in 304, such as trench and via sidewalls. Therefore, the trench andvia sidewalls and vertical surfaces in general are not etched. Surfaceson the semiconductor device which have been pretreated with the inertplasma, however, are activated as described in 304 and exhibit enhancedNH₄F or NH₄F(HF) adsorption. On these plasma activated surfaces NH₄F isadsorbed and NH₄F based etching of silicon oxides is achieved. As such,at temperatures from 65° C. to 100° C., such as from 70° C. to 100° C.,the silicon oxide is not etched in side walls of vias and trenches, andit is etched in activated areas such as the upper surface of thesubstrate and bottoms of trenches and vias.

The etching process described by the method above is also capable ofselectively etching silicon oxide relative to SiN, Si, and metalsilicides. NH₄F etches silicon oxide without substantially etchinglayers, such as silicon nitride or metal silicides. The selectivitybetween SiO₂ and SiN is greater than 5:1 and in some examples greaterthan 9:1. Selectivity of SiO₂ to Si is at least greater than 5:1. Thus,the above method provides for both selectivity and directionality inetching of silicon oxides. Other oxides such as GeO₂ may also be etchedin this manner.

It is also possible to etch some metal oxides by the combination ofplasma activation and NH₄F or NH₄F(HF) exposure. As an example, Nickelsilicide (NiSi) can form mixtures of nickel oxide and silicon oxides onits surface. The plasma activation used in this method can facilitatechemical removal of NiO from the surface by both physical bombardment(sputtering) and by enhanced chemical reaction. Other metal oxides andsilicides may be cleaned in similar manner.

The method described above can also be applied to other semiconductormaterials, such as SiGe. Air exposed SiGe or Ge rapidly forms a surfacelayer of adsorbed carbon containing species that generally degradeetching by NH₄F(HF). Plasma activation of the surface has been shown toremove surface carbon and enhance subsequent surface etching byNH₄F(HF). There are many other examples of materials suitable to usewith the method described with reference to FIG. 2. These examples areintended to be illustrative of a broader class of cleaning applicationsand are not intended to be restrictive.

FIG. 3 depicts a graph 350 of the etch rate of the silicon oxide as afunction of pedestal temperature, according to one embodiment. A siliconsubstrate with a silicon oxide layer formed on the surface waspositioned in a processing chamber and processed at each temperaturelevel. The substrates were positioned on the substrate support andreceived a RF bias of 100 W, where the bias is optimally between 25 Wand 200 W. A low energy (e.g. argon or helium plasma formed at 100 W RFpower) inert plasma was delivered to the biased substrate, pre-treatingonly the horizontal regions of the substrate. Inert plasma refers to aplasma which is not chemically reactive to SiO₂, such as Ar, He, H₂, N₂or combinations thereof.

The pre-treated substrate was then exposed to NH₄F etchant remotelygenerated in the processing chamber at wafer support temperaturesbetween 15° C. and 70° C. Etch rates were measured and plotted as shownin the graph with the oxide etch rate in A/sec. over temperature indegrees C. Treated and untreated regions showed no difference intemperatures between 15° C. and 30° C. From 30° C. to about 62° C. thetreated surface was etched at a higher rate. However, both the treatedand untreated surfaces show a linear decline in etch rate. The declinein etch rate after 30° C. is believed to be related to an increase indesorption of the NH₄F from the surface of the substrate prior to theformation of (NH₄)₂SiF₆. The etch rate is believed to be higher on thepre-treated surface due to the surface changes from the pre-treatmentstep. At temperatures higher that 62° C., the etch rate on the untreatedsurface is substantially lower than either the prior temperatureuntreated surface etch rate or the treated surface etch rate. It isbelieved that, at this temperature or above, the adsorption rate and thedesorption rate are equal. Thus, a minimal amount of (NH₄)₂SiF₆ isformed on the untreated surfaces. The pre-treated surfaces continue toshow a linear decline in etch rate but the treated surface etch ratesare substantially higher than the untreated surface etch rates. Thespecific temperature ranges for enhanced SiO₂ etch rates shown in FIG. 3are based on a specific NH₄F partial pressure and associated NH₄F dewpoint temperature. Processes with higher NH₄F partial pressures canshift their etch rates to higher temperatures than shown in FIG. 3. Itis possible to achieve differential etch rates between plasma treatedand non-plasma treated surfaces to substrate temperatures of 110° C. ormore.

FIGS. 4A-4C are graphical representations of a substrate 500 etchedaccording to one or more embodiments. FIG. 4A depicts a substrate 500with a surface oxide layer 503 according to one embodiment. Thesubstrate 500 can be a silicon-containing substrate, such as acrystalline silicon substrate. The substrate 500 has an upper surface502. The upper surface 502 has a surface oxide layer 503 formed thereon,such as a silicon oxide layer formed on a silicon-containing substrate.The surface oxide layer 503 can be a result of transfer between chambers(i.e., exposure to atmosphere). The substrate 500 can further have viasand trenches formed therein, such as a via 508. The surface oxide layer503 can be sidewall surfaces 506 of features. The substrate 500 can bepositioned in a processing chamber as described above.

FIG. 4B depicts a substrate 500 during plasma pre-treatment according toone embodiment. The substrate 500 is treated with a low energy inertplasma 510, as described with reference to the embodiments above. Theplasma 510 can alter the top and bottom surfaces 512 without alteringthe sidewall surfaces 506. The bias in the substrate 500 providesdirectionality to the low energy inert plasma. The bias delivered to thesubstrate can be between 20 W and 200 W. Plasma treatment of the surfacecan also be achieved by relying on self-bias of the wafer in acapacitively coupled plasma or inductively coupled plasma. The biasingplasma may furthermore be run at RF frequencies from 350 KHz to 60 MHz.The biasing plasma may be pulsed or continuous to further tailor the iondose intensity being delivered to the wafer.

FIG. 4C depicts the substrate 500 after etching with the NH₄F etchant,according to one embodiment. The substrate 500 is heated to a secondtemperature, such as a temperature higher than 62° C. The substrate 500is then treated with the previously formed NH₄F etchant. The NH₄Fetchant adsorbs preferentially into the horizontal (pre-treated)surfaces 512 with very little adsorbing to the vertical (untreated)surfaces 506. After the (NH₄)₂SiF₆ film is formed on and from thehorizontal surfaces 512, the substrate is annealed to sublimate the(NH₄)₂SiF₆ film thus exposing the cleaned surfaces 514. The thicknessand composition of vertical surfaces 506 are substantially unchanged.

It is believed that the high temperature applied to the substrate 500during the NH₄F plasma treatment further assists in etching by enhancingsublimation. As the temperature that the substrate is maintained at isvery close to the temperature which the (NH₄)₂SiF₆ film is believed tosublimate, the film is expected to be partially sublimating as it isformed on the pretreated surfaces. Therefore, less annealing isnecessary while simultaneously exposing more of the surface to NH₄Fetchant.

One alternate implementation captured in this invention is to maintainthe wafer above the sublimation temperature for (NH₄)₂SiF₆ during theNH₄F/NH₄F(HF) exposure process. In this way chemical etch byproducts areformed and removed simultaneously, which can reduce or eliminate theneed for a final substrate anneal step and significantly increase waferprocessing throughput.

FIGS. 5A and 5B show a transmission electron microscope image of asilicon oxide layer precleaned as disclosed in one embodiment. FIG. 5Ashows a silicon substrate with two trenches formed therein. Disposedover the surface of the silicon substrate is a conformal layer ofsilicon oxide approximately 230 Å thick. FIG. 5B shows an identicalstructure after directional etching by one embodiment of inventiondescribed herein. The silicon substrate was pretreated with a low energyinert plasma at a pressure of 100 mTorr of Argon. Conditions for theplasma were selected to avoid sputtering of the silicon oxide layer. Thesubstrate received a 50 W bias at 13.56 Mhz during the plasma treatmentand the plasma was delivered over 10 seconds. Post-inert plasmatreatment, the substrate was exposed to NH₄F formed in a remote plasmasource. The substrate was maintained at a temperature of 75° C. duringthe treatment with NH₄F. The (NH₄)₂SiF₆ film was formed on thepretreated surfaces and sublimated from the surface of the substrateduring a post-treatment anneal at 120° C.

FIG. 5B shows substantially reduced oxide thickness at the top andbottom of the trench structures but almost no change in the thickness ofthe sidewall oxide layers. The bottom etch amount was 91 Å which closelycorrelates to the slightly higher etch amount of 106 Å on the topsurfaces. The side walls were etched from 5 Å at the top side wall to 0Å at the mid side wall. The determined etch rates show at least a 10:1preferential etching of the top and bottom surfaces over the sidewallsurfaces.

CONCLUSION

Embodiments described herein relate to methods of directional removal ofnative oxides form a surface. Above embodiments show preferentialetching of pretreated surfaces over untreated surfaces. SiO₂ is formedby various means on silicon, SiGe, SiC, and various metal silicidesurfaces and must be removed for proper deposition in vias and trenches.However, it is important to avoid changing the cross-sectionaldimensions of the modern day vias and trenches, which can lead to devicefailure. By pretreating a biased substrate with a low energy inertplasma, the horizontal surfaces will be modified either physically or bychange in the type or bonding energy of available surface sites. Thus, achemical etch by NH₄F plasma at a high temperature will be effective forremoving the pre-treated surfaces without affecting the untreatedsurfaces on the side walls of the vias or trenches.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method, comprising: positioning a substrate ina processing chamber, the substrate comprising: an exposed surfacehaving one or more features formed in the exposed surface, the featurescomprising a bottom surface; and an oxide layer formed on the exposedsurface; biasing the substrate; exposing the substrate to an inertplasma to selectively form an activated material on the oxide layer onthe exposed surface and the bottom surface of the features; heating thesubstrate to a first temperature; exposing the substrate to a processinggas comprising ammonium fluoride (NH₄F) or NH₄F(HF) to form one or morevolatile products on the activated material; and heating the substrateto a second temperature, which is higher than the first temperature, tosublimate the one or more volatile products.
 2. The method of claim 1,wherein the first temperature and the second temperature are within 10°C. of one another.
 3. The method of claim 1, wherein the firsttemperature is a temperature of at least 65° C.
 4. The method of claim1, wherein the first temperature is a temperature of between 70° C. and90° C.
 5. The method of claim 1, wherein the second temperature is atemperature of greater than 100° C.
 6. The method of claim 1, whereinthe first and second temperatures are both greater than 100° C., andwherein simultaneous etching and sublimation of etch byproducts occurs.7. The method of claim 1, wherein heating the substrate to a secondtemperature further comprises exposing the exposed surface of thesubstrate to a low energy inert plasma.
 8. The method of claim 1,wherein a plasma is formed from the processing gas, and wherein theplasma is formed remotely.
 9. The method of claim 8, wherein the plasmacomprising ammonium fluoride is a non-sputtering plasma.
 10. The methodof claim 1, wherein the processing gas is a gas mixture comprisingammonia (NH₃) and nitrogen trifluoride (NF₃).
 11. The method of claim10, wherein the gas mixture is a 1:1 or higher ratio of ammonia (NH₃)and nitrogen trifluoride (NF₃) respectively.
 12. The method of claim 11,wherein the ratio of NH₃ to NF₃ is 5:1 or higher.
 13. The method ofclaim 1, wherein NH₄F is formed in a plasma remote from the processingchamber from a formation gas comprising NH₃, NF₃, HF, F₂, H₂, He, Ar orcombinations thereof.
 14. The method of claim 1 where NH₄F is formed ina plasma inside the processing region of the processing chamber from aformation gas comprising NH₃, NF₃, HF, F₂, H₂, He, Ar or combinationsthereof.
 15. The method of claim 1, further comprising: maintaining thesubstrate at a first temperature during the exposure to the processinggas; moving the substrate to a second chamber; and sublimating one ormore etch byproducts.
 16. The method of claim 1, wherein the exposingthe substrate to an inert plasma, the heating the substrate to a firsttemperature, the exposing the substrate to NH₄F, NH₄F(HF) orcombinations thereof, and heating to a second temperature are repeatedone or more times.
 17. A method comprising: positioning asilicon-containing substrate in a processing chamber, thesilicon-containing substrate comprising: an exposed surface; one or morefeatures formed in the exposed surface; and a layer of surfacecontaminants formed on the exposed surface; heating the substrate to afirst temperature; exposing the layer of surface contaminants on theexposed surface of the substrate to ammonium fluoride (NH₄F), (NH₄F)HF,HF or combinations thereof at the first temperature; biasing thesubstrate; exposing the substrate to an inert plasma to selectively formone or more volatile products on the exposed surface and bottom surfacesof the features; exposing the substrate to low pressure at a secondtemperature, wherein non-reacted NH₄F is sublimated from the exposedsurface of the substrate; and heating the substrate to a thirdtemperature, which is higher than the first temperature and the secondtemperature, to sublimate the one or more volatile products.
 18. Themethod of claim 17, wherein the exposed surface is exposed to a gasmixture comprising ammonia (NH₃) and nitrogen trifluoride (NF₃), andwherein the gas mixture is a 1:1 or higher ratio of ammonia (NH₃) andnitrogen trifluoride (NF₃) respectively.
 19. The method of claim 17,wherein NH₄F is formed in a plasma remote from the processing chamberfrom a formation gas comprising NH₃, NF₃, HF, F₂, H₂, He, Ar, orcombinations thereof.
 20. A method, comprising: positioning asemiconductor substrate in a processing chamber, the semiconductorsubstrate comprising: an exposed surface having one or more featuresformed in the exposed surface; and an oxide layer formed on the exposedsurface; biasing the substrate; performing a contaminant removalprocess, comprising: exposing the substrate to an inert plasma toselectively form an activated material on the oxide layer on the exposedsurface and bottom surfaces of the features; heating the substrate to afirst temperature; exposing the surface of the substrate to a processinggas comprising ammonium fluoride (NH₄F) or NH₄F(HF) to form one or morevolatile products on the oxide layer on the exposed surface and bottomsurfaces of the features; and heating the substrate to a secondtemperature, which is higher than the first temperature, to sublimatethe one or more volatile products; and repeating the contaminant removalprocess one or more times.